Waveguide filter having sequence of thick capacitive irises

ABSTRACT

A compact microwave band-pass filter utilizes a sequence of thick capacitive irises stationed along the interior of a waveguide to form a series of directly coupled resonant chambers. The spacing between consecutive irises is lambda g /4 or less, where lambda g is the upper cutoff frequency of the filter. The close iris spacing causes the second harmonic pass band for the fundamental mode to be far above the main pass band.

ited States Patent Inventor Ralph Levy Newton, Mass.

Appl. No. 786,967

Filed Dec. 26, 1968 Patented May 4, 1971 Assignee Microwave DevelopmentLaboratories, Inc.

Needham Heights, Mass.

WAVEGUIDE FILTER HAVING SEQUENCE OF THICK CAPACITIVE [RISES 1 Claim, 12Drawing Figs.

US. Cl 333/73, 3 33/76 Int. Cl H03h 7/10 Field of Search 333/76;

[56] References Cited UNITED STATES PATENTS 2,740,094 3/ 1956 Fox 333/732,787,766 4/ 1957 Scheftelowitz 333/73 3,027,525 3/1962 Salzberg 333/733,196,339 7/1965 Walker et a]. 321/69 Primary ExaminerEli LiebermanAttomey-Louis Orenbuch ABSTRACT: A compact microwave band-pass filterutilizes a sequence of thick capacitive irises stationed along theinterior of a waveguide to form a series of directly coupled resonantchambers. The spacing between consecutive irises is A, /4 or less, whereA is the upper cutoff frequency of the filter. The close his spacingcauses the second harmonic pass band for the fundamental mode to be farabove the main pass band.

PATENTEMAY 4:911 3577.104

SHEET 2 of 4 FIG. 4A

INVENTOR RALPH LEVY ATTORNEY 357K104 saw u or 4 8P) NOliVflNHliVPATENTEU HAY 4 I911 3 INVENTOR BY RALPH LEVY ATTORNEY OILVE HAVMJNICINVJS (HMS/U WAVEGUmE FILTER HAVING SEQUENCE OF THICK CAPACITIVEIRISES This invention relates to frequency selective passive apparatusfor filtering signals which are in the microwave portion of theelectromagnetic frequency spectrum. More particularly, the inventionpertains to a structure utilizing thick, capacitive irises which areclosely spaced along the interior of a waveguide to form a compactband-pass filter that exhibits good harmonic rejection.

DISCUSSION OF THE PRIOR ART Frequency filters formed by capacitiveirises spaced along the interior of a waveguide to form a sequence ofresonant cavities are described in Maximally Flat Filters in Waveguide,by W. W. Mumford, Bell System Technical Journal, Vol. 27, Oct. 1948 andin Design of Tunable Resonant Cavities With Constant Bandwid by L. D.Smullen, Proc. IRE, Vol. 37, Dec. 1949. The capacitive iris waveguidefilters are principally considered to be variants of the more commonfilters having inductive irises in the waveguide which form resonantcavities that are each approximately M2 in length, where )t, is thewavelength in the guide. The resonant cavities in inductive iris filtersare somewhat shorter than 180 in electrical length (that is, somewhatless than ).,,/2) whereas the resonant cavities in a capacitive irisfilter are somewhat longer than 180. In the capacitive iris filter, thebandwidth remains fairly constant when the filter is tuned over a widefrequency range, and the rate of cutoff is more rapid at the highfrequency edge of the band compared to the low frequency band edgecutoff rate. For inductive iris filters, the reverse situation isencountered both for the cutoff rate at the band edges and for tuningover a wide frequency range. The disadvantages of a half-wavelengthcapacitive iris filter arise from the difficulty of producing largecapacitive susceptances in waveguide. Because of that difficulty, astructure employing capacitive irises to form direct-coupled resonantcavities is not generally deemed to be a feasible type of narrow bandfilter and in a capacitive iris, direct-coupled, resonant cavity,waveguide filter of moderate bandwidth the harmonic rejection capabilityis little better than in the inductive iris filter because the resonantcavities are long and consequently possess harmonic windows (viz,harmonic pass bands) at comparatively low frequencies above the mainpass band.

Where two irises form a resonant cavity in a conventional waveguidefilter, the shortest length of the cavity is between one-half tothree-quarters of the guide wavelength A It is known from theoreticalconsiderations that the spacing between the capacitive irises may bebetween zero and onefourth A, but the interacting fringing effectsassociated with the close spacing of the two irises is deemed to makesuch close spacing impracticable. See, for example, the discussion inPrinciples and Applications of Waveguide Transmission, by George C.Southworth, pp. 286 to 287, published by Van Nostrand.

In US. Pat. No. 3,027,527, there is disclosed a waveguide band-passfilter, invented by Edward Salzberg, which has capacitive irises spacedalong the waveguide at one-quarter wavelength (A14) intervals. In theSalzberg filter, a resonant iris is situated midway between adjacentcapacitive irises. The reduction thus achieved in the length of thecavity has resulted in an improvement in harmonic rejection in additionto providing a more compact structure.

THE INVENTION The invention here disclosed provides better performancethan can be obtained with the Salzberg arrangement while retaining thecompactness of the filter. In the invention, thick capacitive irises aredisposed along the interior of a waveguide to form a series of directlycoupled resonant chambers. The spacing between consecutive irises isapproximately between one-quarter (V4) to one-eighth (Vs) of M thewavelength in the guide at the upper cutoff frequency of the filter.Because formance of the filter can be realistically predicted by simplecascade analysis.

THE DRAWINGS The invention, both as to its construction and mode of use,can be better understood from the following exposition when consideredin conjunction with the accompanying drawings in which:

FIG. I symbolizes a cavity formed in a waveguide by capacitive irisesspaced apart by an electrical length 9;

FIG. 2A is a diagram of a filter having n directly coupled cavitiesformed by a sequence of capacitive irises spaced along a waveguide;

FIG. 2B symbolizes the distributed low-pass prototype of FIG. 2A inimpedance inverter form;

FIG. 3 represents an n-cavity filter with generalized symmetricallossless networks;

FIGS. 4A and 4B depict a thick capacitive iris in a rectangularwaveguide;

FIG. 4C symbolizes the equivalent electrical circuit of the thickcapacitive iris;

FIG. 5 depicts various capacitive iris configurations;

FIG. 6 is a view of an embodiment of the invention with part of thewaveguide broken away to expose capacitive irises;

FIG. 7 is a sectional view taken along the plane 7-7 in FIG.

FIG. 8 is a top view of the FIG. 6 embodiment with the upper broad wallof the waveguide removed to expose the interior of the filter; and

FIG. 9 is a graph comparing actual filter performance with theoreticallypredicted performance.

THE EXPOSITION FIG. 1 symbolically depicts a single cavity formed by apair of capacitive irises l and 2 spaced in a waveguide 3. Eachcapacitive iris is characterized by a normalized susceptancejb and theirises are spaced by an electrical length 6. The insertion loss of thecavity is given by For expository purposes, the filters consideredherein have been given susceptance values in the range 0.5 'to 4 and thecorresponding resonant lengths are therefore in the range approximately25 to 75. The second harmonic resonances consequently are in the 205 to255 range which is far removed from the fundamental range.

Consider now the waveguide filter in FIG. 2A having n cavities formed bycapacitive irises spaced along the interior of the waveguide to formcavities that are 11/2 or less in electrical length. The theorypresented in my monograph Theory of Direct-Coupled Cavity Filters," IEEETransactions on Microwave Theory and Techniques, Vol. MTT-IS, No. 6 June1967, is not completely applicable here because the cavities in the FIG.2A filter are appreciably less than a halfwavelength and that filter islow-pass rather than band-pass. The susceptances of the irises in theFIG. 2A filter are, however, determined in a manner similar to that setforth in the aforesaid monograph. The susceptances are determined byequating the VSWR (the voltage standing wave) ratio of each reactivecoupling discontinuity of the filter (i.e., each iris) to thecorresponding junction VSWR V of a distributed low-pass prototypefilter. See my monograph Table of Element Values For the Distributedbow-Pass Prototype Filter, IEEE Transactions on Microwave Theory andTechnique, Vol. MIT-l3, No. 5, Sept. 1965. FIG. 2B shows the distributedlow-pass prototype filter of FIG. 2A in impedance inverter form. For adiscussion of the concept of the impedance inverter see Direct-CoupledResonator Filters" by S. B. Cohn, Proc. IRE, Vol. 45, Feb. 1957. TheFIG. 2B prototype leads to the equation b (i=1, 2, n+1) (3) This givesan exact realization of the upper cutoff frequency of the filter, if thesusceptance values are chosen to satisfy equation (3) at that particularfrequency and if the spacings between irises are properly chosen. Thespacings between irises should all be equivalent to an effectiveelectrical length of 6,, at the upper cutoff frequency. The actualelectrical length between any pair of adjacent irises is greater than 6because it necessary to measure the effective spacings from referenceplanes on either side of the irises as indicated in FIG. 2A to accountfor the phase shift introduced by the irises.

The transfer matrix of any iris of normalized susceptancejb shunting aline of electrical length 1 at its midpoint is given by cos -%b sin 5This acts as an ideal impedance inverter if the condition 2 '"l 4: tan bis satisfied, resulting in the transfer matrix 0 jk] i k 0 (6) where:

7) and the application of equation (3) shows that The reference planesin Fig. 2A are therefore spaced from each iris b, where:

=tarr The actual electrical length 5, between irises z' and i+ 1 isgiven by Equation (9) can be extended to cover the case of capacitiveirises whose thickness is such that the irises cannot be represented bysimple shunt susceptances. The extension covers the general caserepresented by the filter shown in FIG. 3 which has n cavities and inwhich each iris has been replaced by a network T, which may be anysymmetrical reciprocal lossless two-port network. The transfer matrix ofany such network T, including adjacent lengths of line I /2 on eitherside is given by The condition that this shall be equivalent to arfinverter, as in equation (6), is that is the insertion loss of thesymmetrical two-part network when terminated in a normalizedcharacteristic impedance of unity. In obtaining this result, thereciprocity condition A +BC=l (14) has been used and it should be notedthat equation (12) may be reduced to the form given by equation (8).When T is a simple shunt susceptancejb, as in FIG. 2A, then M 1 8 0; C=b1 5 and equation (11) reduces to equation (5), as required. At thispoint, it is convenient to introduce the concept of an equivalent shuntsusceptance b, defined as having a value giving the same insertion lossas that given by equation (13) for the general network. We have,therefore,

or F= B c (16) where B and C are normalized to the terminatingimmitance.

FIG. 4A depicts a thick capacitive iris in a rectangular waveguide andFIG. 4B is a sectional view taken along the parting plane 48-48 in FIG.4A. In FIG. 4A, the narrow internal dimension of the waveguide isdenoted by b in accordance with conventional notation and should not beconfused with a normalized susceptance. The equivalent circuit of thethick capacitive iris is symbolized in FIG. 4C and the parameters ofthat circuit are given in pages 250 to 255 of the Waveguide Handbook,MIT Radiation Laboratory Series, Vol. 10, published by McGraw Hill. Withslight changes in nomenclature, the parameters are as follows and g is aparameter lying in the range l g l .67 for and is plotted on page 253 ofthe Waveguide Handbook with ht. A sufficiently accurate value of g for agiven t/d may be obtained from linear interpolation between the specificpoints in the following table.

TABLE 2 Cavity 1 2 3 4 5 6 Length 403 322 285 285 .322 403 As the designis physically symmetrical the cavity at either end can be considered asthe first cavity. In the embodiment, two types of capacitive irises areemployed alternately. The first The design of a filter employing thickcapacitive irises proceeds by applying the generalized impedanceinverter theory. The transfer matrix of the thick iris equivalentcircuit (FIG. 4C) is so that the equivalent normalized susceptance asdefined by equation (16) is where b is related to the correspondingjunction VSWR of the distributed low-pass filter by the equation 1 aw T22) FIG. 4A is merely representative of one iris configuration that iscapacitive, i.e., which is characterized by positive susceptance. Othercapacitive iris configurations commonly used in rectangular waveguideare shown in FIG. 5. The invention is applicable to capacitive irises ingeneral and is not restricted, to any particular iris configuration.

FIG. 6 depicts an embodiment of the invention with parts of thewaveguide broken away to show the capacitive irises.

FIG. 7 is a sectional view taken along the parting plane 7-7 in FIG. 6and Hg. 8 is a top view of the invention with the upper broad wall ofthe waveguide removed to expose the interior of the filter. In theillustrated embodiment, the waveguide 10 is partitioned by thecapacitive irises 1, 2,...7 to provide six cavities. In an actualembodiment of the invention that was constructed, the six-cavity filterwas designed for an upper cutoff frequency of 7650 MHz. and was based ona distributed low-pass prototype having N=6, VSWR=1.05 and BW 0.50 (seemy monograph Table of Element Values For The Distributed Low-PassPrototype Filter, cited supra). In that actual embodiment WR 137waveguide was employed, the inside dimensions of the waveguide beinga=l.372 and b=0.622. The spacings measured in inches between facingsurfaces of adjacent irises are shown in FIg 8 and are tabulated belowas cavity lengths iris type is a conductive plate which extends acrossthe guide to form an aperture with the top waveguide broad wall andanother aperture with the bottom waveguide broad wall. The second typeof capacitive iris employs a pair of aligned conductive plates whichextend from the top and bottom broad walls of the waveguide and have agap between them. The first type is represented in FIG. 5 by the plate51 while the second type is represented in that FIG. by plates 52 and53.

From FIG. 7 it can be seen that irises 1, 3, 5, and 7 are of the firsttype and irises 2, 4, and 6 are of the second type. All the irises aresymmetrically disposed in the interior of the waveguide, as thatdisposition is a manufacturing convenience. All the iris plates arepreferably rectangular in cross section and in the example heredescribed are 0.062 inch thick. For irises 2 and 6, the gap between the0.062 inch thick plates is 0.192 inch and for iris 4 the gap between the0.062 inch thick plates is 0.118 inch. The height of the plate formingiris 1 or 7 is 0.255 inch and the height of the plate forming iris 3 or5 is 0.493 inch. The filter has no tuning screws although such screwscan be provided if desired.

FIG. 9 is a graphical comparison between the theoretically predictedresults, computed by multiplication of transfer matrices, and theresults obtained from actual embodiments of the six-cavity filter. Thetheoretical VSWR is reproduced quite well at frequencies below 7.3 GHz.,but the insertion loss curve shows that the upper cutoff frequency F is600 MHz. higher than predicted. The cutoff rate is in good agreementwith theory, and the attenuation in the stop band above the upper cutoffwas ascertained to be greater than 40 db. to 16 GHz. and greater than 25db. to 18 GHz. Closer agreement between theoretical predicted and actualresults has been obtained for several later filter desigis and it hasbeen found that in the later designs the theoretical upper cutofifrequency is usually more closely approached.

The order of magnitude of the interacting effects arising from the closespacing of the irises can be estimated by calculating the proximityfactor P by which the adjacent capacitances must be multiplied to obtaina more correct result, as described, for example, by H. E. Green in TheNumerical Solution of Some Important Transmission Line Problems, IEEETrans. on Microwave Theory and Techniques, Vol. MIT-l3, Sept. 1965. Ithas been found that even for the shortest cavity in the describedsix-cavity filter, the proximity factor is greater than 0.9 and hencethe proximity effect is small. Much of the capacitance of a thick irisis due to the thickness itself, that is, is due to the parallel platecapacitance of the facing surfaces, and as the parallel platecapacitance is not affected by the proximity factor, the mutualinteracting effects of the closely spaced thick irises need not, in manyinstances be considered.

A thick capacitive iris, within the context of this exposition, means aniris whose thickness is such that the series inductance of the iris isnot negligible compared with its shunt capacitance. In terms of thelumped equivalent circuit of a thick iris, the series inductivecomponent is appreciable compared with the shunt capacitive component.

The lower cutoff frequency of the filter is determined by the normalcutoff frequency of the waveguide. The filter may, therefore, beconsidered a low-pass filter in the sense that of of or less, where M isthe wavelength in the guide at the upper cutoff frequency of the filterand the distance is measured between facing surfaces of the adjacentirises, and

the actual electrical length 6, between adjacent irises i and i+1 is6-=0 +V tantan 5 l 0 2 i i+1 I where 6,, is the effective electricallength at the upper cutoff frequency between reference planes ofadjacent irises, and

b Vi W.

where V, is the corresponding junction VSWR of a distributed 1 5low-pass prototype filter.

1. In a microwave band-pass filter having a sequence of irisespositioned in a waveguide to form a series of directly coupled resonantcavities, the improvement wherein each iris is a capacitive iris whosethickness is such that within the pass band of the filter the seriesinductance of the iris is appreciable compared to its shunt capacitance,adjacent irises are spaced alOng the waveguide by a distance of lambda g/4 or less, where lambda g is the wavelength in the guide at the uppercutoff frequency of the filter and the distance is measured betweenfacing surfaces of the adjacent irises, and the actual electrical lengthTheta i between adjacent irises i and i+ 1 is where Theta o is theeffective electrical length at the upper cutoff frequency betweenreference planes of adjacent irises, and where Vi is the correspondingjunction VSWR of a distributed low-pass prototype filter.